What do you mean by "Windows display resolution"? A setting at Start Menu/Settings/Display? (I haven't found any that have an influence on the thickness of the underlining)
Anyway, I don't think it's a Windows problem, but rather an Office problem, because as I said:
- I have the same problem in PowerPoint.
- I do not have the problem in my mailing software (Thunderbird) where the underlining is normal (thicker).
- If I make a screen copy of the Word page where the underlining is abnormally unseen, in the screen copy the underlining is normal (visible)